1. Technical Field of the Invention
The invention relates to the field of electrically controllable displays, and more particularly to the field of electrochromic displays.
2. Description of the Prior Art
There are many uses for electrically controllable display devices. A number of such devices have been in commercial use for some time. These display devices include liquid crystal displays, light emitting diode displays, plasma displays and so on. Light emitting diode and plasma display panels both suffer from the fact that they are active, light emissive devices which require substantial power for their operation. In addition, it is difficult to fabricate light emitting diode displays in a manner which renders them easily distinguishable under bright ambient illumination. Liquid crystal displays suffer from the disadvantage that they are operative only over a limited temperature range and have substantially no memory within the liquid crystal material. Further, the visibility of many liquid crystal displays decreases as the viewer moves a few degrees off axis.
Electrochromic displays have been developed which display information through a change in the color of portions in the display via electrochemical reaction of an active material to achieve a color change. Generally with a metal oxide as the active material, this color change is from white to a color such as blue. Because of their coloring mechanism, such displays usually require substantial power and time to write or erase displayed information. The quantities of power required are undesirably large, especially for battery operation. Moreover, the time required to change displayed information makes such materials unacceptable for many display applications. None of these displays display more than a single color against a background. This limits the versatility of such displays since the color of a character cannot be used to convey information.
Rare earth diphthalocyanines are known from prior publications to have electrochromic properties in which the color of the diphthalocyanine can change over a period of about eight seconds upon application of a potential difference across an electrochemical cell having a diphthalocyanine film on one of the electrodes. P. N. Moskalev and I. S. Kirin, "Effect of the Electrode Potential on the Absorption Spectrum of a Rare-Earth Diphthalocyanine Layer," Opt. i Spektrosk, 29, 414 (1970) and P. N. Moskalev and I. S. Kirin, "The Electrochromism of Lanthanide Diphthalocyanines" Russian J. Phys. Chem., 45, 1019 (1972).
U.S. Pat. No. 4,184,751 of M. M. Nicholson describes the use of metal diphthalocyanine complexes as the electrochemically active material in an electrochromic display cell. Rapid color changes in less than 50 milliseconds are achieved, thus alleviating the slow switching time previously reported for rare earth diphthalocyanine complexes. Power requirements are small because of the lower power switching characteristics of the display material and because the display exhibits an open circuit memory of from several minutes to several hours, depending on its construction. A multi-color, i.e., more than one color, display is achieved through use of a range of voltages applied between display and counter electrodes. Color reversal of displayed information and the background against which it is displayed is achieved through use of display electrodes in the background portions of the viewing area as well as in the character segments.
The disadvantage of the metal diphthalocyanine complex electrochromic display is that after repeated use the display electrode deteriorates. Studies have in fact shown that under certain experimental conditions the cycle life may be limited to approximately 50,000 cycles. Prior to the present invention, there has not been a metal diphthalocyanine complex electrochromic display with a long cycle life.